U.S. patent application number 16/306779 was filed with the patent office on 2019-05-02 for racket and grommet.
The applicant listed for this patent is YONEX KABUSHIKI KAISHA. Invention is credited to Hitoshi KATO, Masato KAWABATA, Naoto OGAWA, Tsutomu TAKAHASHI.
Application Number | 20190126106 16/306779 |
Document ID | / |
Family ID | 60479334 |
Filed Date | 2019-05-02 |
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United States Patent
Application |
20190126106 |
Kind Code |
A1 |
OGAWA; Naoto ; et
al. |
May 2, 2019 |
RACKET AND GROMMET
Abstract
A racket includes: a grip; an annular frame; and a shaft
coupling the grip and the frame together; wherein a projection is
provided to an outer peripheral face on a leading end half of the
frame in a predetermined range including a location of maximum
curvature in a peripheral direction.
Inventors: |
OGAWA; Naoto; (Niigata,
JP) ; KAWABATA; Masato; (Niigata, JP) ; KATO;
Hitoshi; (Niigata, JP) ; TAKAHASHI; Tsutomu;
(Niigata, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
YONEX KABUSHIKI KAISHA |
Tokyo |
|
JP |
|
|
Family ID: |
60479334 |
Appl. No.: |
16/306779 |
Filed: |
April 27, 2017 |
PCT Filed: |
April 27, 2017 |
PCT NO: |
PCT/JP2017/016711 |
371 Date: |
December 3, 2018 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
A63B 2049/0217 20130101;
A63B 60/006 20200801; A63B 49/02 20130101; A63B 49/022 20151001;
A63B 2102/02 20151001; A63B 2102/06 20151001; A63B 2102/04
20151001 |
International
Class: |
A63B 49/022 20060101
A63B049/022 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 3, 2016 |
JP |
2016-111630 |
Claims
1. A racket comprising: a grip; an annular frame; and a shaft
coupling the grip and the frame together; wherein a projection is
provided to an outer peripheral face on a leading end half of the
frame in a predetermined range including a location of maximum
curvature in a peripheral direction.
2. The racket according to claim 1, wherein when the frame is
viewed as a clock face with a leading end of the frame at 12
o'clock, the predetermined range is a range from 1 o'clock to 2
o'clock and a range from 10 o'clock to 11 o'clock.
3. The racket according to claim 1, wherein the projection has a
width in the peripheral direction wider at a position at a central
side of the projection in a thickness direction, which is
orthogonal to a hitting face formed inside the frame, than at a
position at an end side of the projection in the thickness
direction.
4. The racket according to claim 1, wherein a height of the
projection is 0.5 mm or lower.
5. The racket according to claim 1, wherein: a grommet is attached
to an outer periphery of the frame; and the grommet includes a
recess configured to fit together with the projection of the frame
on a back face of the grommet, which is a side of the grommet
opposing the frame, and includes a protrusion formed to correspond
to the recess on a front face of the grommet, which is the opposite
side to the back face.
6. A grommet for attachment to an outer peripheral face of a frame
of a racket that includes a grip, an annular frame, and a shaft
coupling the grip and the frame together, the grommet comprising: a
protrusion provided to a front face of the grommet at a site
disposed in a predetermined range including a location of maximum
curvature in a peripheral direction of the frame.
7. The grommet according to claim 6, wherein: a projection is
provided on an outer peripheral face of the frame in the
predetermined range; and the grommet includes a recess configured
to fit together with the projection provided on a back face of the
grommet.
8. The racket according to claim 2, wherein the projection has a
width in the peripheral direction wider at a position at a central
side of the projection in a thickness direction, which is
orthogonal to a hitting face formed inside the frame, than at a
position at an end side of the projection in the thickness
direction.
9. The racket according to claim 3, wherein a height of the
projection is 0.5 mm or lower.
10. The racket according to claim 4, wherein: a grommet is attached
to an outer periphery of the frame; and the grommet includes a
recess configured to fit together with the projection of the frame
on a back face of the grommet, which is a side of the grommet
opposing the frame, and includes a protrusion formed to correspond
to the recess on a front face of the grommet, which is the opposite
side to the back face.
Description
RELATED APPLICATIONS
[0001] This application is the U.S. National Phase under 35 U.S.C.
.sctn. 371 of International Application No. PCT/JP2017/016711 A1,
filed Apr. 27, 2017, which in turn claims the benefit of Japanese
Application No. 2016-111630, filed Jun. 3, 2016, the contents of
which are incorporated herein by reference in their entirety.
TECHNICAL FIELD
[0002] The present invention relates to a racket and a grommet.
BACKGROUND ART
[0003] Known rackets used for tennis and the like include a grip,
an annular frame, and a shaft coupling the grip and the frame
together. Grommets are also attached to an outer peripheral face of
the frame (see, for example, Patent Literature 1). Generally,
racket frames have a substantially elliptical shape elongated in
the longitudinal direction as in Patent Literature 1, with a site
of large curvature present at a leading end side of the frame (i.e.
on the opposite side to the grip side). Curvature is the inverse of
radius of curvature, with the greater the curvature (the smaller
the radius of curvature) indicating more bending.
CITATION LIST
Patent Literature
[0004] Patent Literature 1: JP 2009-165703A
SUMMARY OF INVENTION
Technical Problem
[0005] When a frame (and in particular a leading end side thereof)
includes a site with a large curvature, as in the racket described
above, an airflow (flow of air) along an outer peripheral face
thereof is prone to separating during a swing, which may induce
greater air resistance. Furthermore, although there are particular
demands to raise rigidity in the frame at this site, providing a
member to raise rigidity thereat might lead to an increase in
weight.
[0006] In consideration of the above issues, an objective of the
invention is to achieve a reduction in air resistance acting on a
racket during a swing and to also achieve improved rigidity while
suppressing an increase in weight.
Solution to Problem
[0007] A main aspect for achieving the objective is a racket
including: a grip; an annular frame; and a shaft coupling the grip
and the frame together; wherein a projection is provided to an
outer peripheral face on a leading end half of the frame in a
predetermined range including a location of maximum curvature in a
peripheral direction.
[0008] Other features of the invention will be made clear in the
specification and drawings.
Advantageous Effects of Invention
[0009] The racket of the invention is able to achieve a reduction
in air resistance acting on the racket during a swing and is also
able to achieve improved rigidity while suppressing an increase in
weight.
BRIEF DESCRIPTION OF DRAWINGS
[0010] FIG. 1A is a front view of a racket, and FIG. 1B is a side
view of the racket.
[0011] FIG. 2A is a perspective view of a frame of Comparative
Example 1. FIG. 2B and FIG. 2C are cross-sections of the frame of
Comparative Example respectively taken at position aa and position
bb in FIG. 2A. FIG. 2D is a diagram illustrating an inner
peripheral face of the frame of Comparative Example 1, as viewed
along a direction of penetration.
[0012] FIG. 3A and FIG. 3B are diagrams to explain an airflow
passing over the frame of Comparative Example 1.
[0013] FIG. 4A and FIG. 4B are perspective views of a frame of
Comparative Example 2. FIG. 4C is a cross-section of a frame at
position aa in FIG. 4A.
[0014] FIG. 5A is a cross-section of the frame at position bb in
FIG. 4A. FIG. 5B is diagram illustrating an inner peripheral face
of the frame as viewed along a penetration direction. FIG. 5C is a
cross-section of the frame at position aa in FIG. 5B.
[0015] FIG. 6A and FIG. 6B are diagrams to explain an airflow
passing over the frame of Comparative Example 2.
[0016] FIG. 7A is a front view of a racket 1 of an embodiment, and
FIG. 7B is a side view of the racket 1 of the present
embodiment.
[0017] FIG. 8 is an explanatory diagram of a grommet 50 of the
present embodiment.
[0018] FIG. 9 is a diagram illustrating a cross-section taken along
A-A in FIG. 8 and a cross-section taken along B-B in FIG. 8.
[0019] FIG. 10A is a front view of a leading end portion of the
racket 1 of the present embodiment in a state in which the grommet
50 has been attached to a frame 10 of the racket 1, and FIG. 10B is
a perspective view thereof.
[0020] FIG. 11A and FIG. 11B are cross-sections taken along A-A in
FIG. 10A. FIG. 11A is a cross-section illustrating the frame 10
alone, and FIG. 11B is a cross-section illustrating a state in
which the grommet 50 has been attached to the frame 10.
[0021] FIG. 12A is a concept diagram illustrating a flow of air at
the outside of a frame 10' of Comparative Example 1. FIG. 12B is a
concept diagram illustrating a flow of air at the outside of the
frame 10 (with the grommet 50 attached) in the present
embodiment.
[0022] FIG. 13A and FIG. 13B are diagrams to explain a test method
to evaluate air resistance acting on the racket 1.
[0023] FIG. 14A and FIG. 14B are diagrams illustrating the results
of evaluation tests.
DESCRIPTION OF EMBODIMENTS
Overview of Disclosure
[0024] At least the below matters will become clear from
descriptions of this specification and drawings.
[0025] A racket will become clear including: a grip; an annular
frame; and a shaft coupling the grip and the frame together;
wherein a projection is provided to an outer peripheral face on a
leading end half of the frame in a predetermined range including a
location of maximum curvature in a peripheral direction.
[0026] Such a racket is able to achieve a reduction in air
resistance acting on the racket during a swing and also to achieve
improved rigidity while suppressing an increase in weight.
[0027] According to the racket, wherein preferably when the frame
is viewed as a clock face with a leading end of the frame at 12
o'clock, the predetermined range is a range from 1 o'clock to 2
o'clock and a range from 10 o'clock to 11 o'clock.
[0028] Such a racket is able to raise rigidity at sites on the
frame with a large curvature and is also able to achieve a
reduction in air resistance.
[0029] According to the racket, wherein preferably the projection
has a width in the peripheral direction wider at a position at a
central side of the projection in a thickness direction, which is
orthogonal to a hitting face formed inside the frame, than at a
position at an end side of the projection in the thickness
direction.
[0030] Such a racket is able to achieve a reduction in air
resistance irrespective of the angle of the racket during a
swing.
[0031] According to the racket, wherein preferably a height of the
projection is 0.5 mm or lower.
[0032] Such a racket is able to suppress separation of airflow and
is able to achieve a reduction in air resistance at the frame outer
peripheral side.
[0033] According to the racket, wherein preferably: a grommet is
attached to an outer periphery of the frame; and the grommet
includes a recess configured to fit together with the projection of
the frame on a back face of the grommet, which is a side of the
grommet opposing the frame, and includes a protrusion formed to
correspond to the recess on a front face of the grommet, which is
the opposite side to the back face.
[0034] Such a racket is able to achieve a reduction in air
resistance even when the grommet is attached to the frame.
[0035] Further, a grommet for attachment to an outer peripheral
face of a frame of a racket that includes a grip, an annular frame,
and a shaft coupling the grip and the frame together will become
clear, the grommet including: a protrusion provided to a front face
of the grommet at a site disposed in a predetermined range
including a location of maximum curvature in a peripheral direction
of the frame.
[0036] Such a grommet is able to achieve a reduction in air
resistance easily.
[0037] According to the grommet, wherein preferably: a projection
is provided on an outer peripheral face of the frame in the
predetermined range; and the grommet includes a recess configured
to fit together with the projection provided on a back face of the
grommet.
[0038] Such a grommet is able to achieve a reduction in weight.
===Basic Racket Configuration===
[0039] A racket according to the invention will now be described
for an embodiment in which a tennis racket is given as an example
thereof.
[0040] FIG. 1A is a front view of a racket 1. FIG. 1B is a side
view of the racket 1. The racket 1 includes a grip 30, an annular
frame 10 (generally a substantially elliptical shaped frame
elongated in the longitudinal direction), and a shaft 20 coupling
the grip 30 and the frame 10 together. In the following
explanation, a direction on the frame 10 in which the grip 30 and
the shaft 20 are coupled together is referred to as the
"longitudinal direction", a direction along a hitting face formed
at the inside of the frame 10 and orthogonal to the longitudinal
direction is referred to as the "lateral direction", and a
direction orthogonal to both the longitudinal direction and the
lateral direction (i.e. a direction orthogonal to the hitting face)
is referred to as the "thickness direction". The side of the frame
10 where the shaft 20 is positioned is referred to as the
"longitudinal direction rear end side", and the opposite side
thereto is referred to as the "longitudinal direction leading end
side".
[0041] Plural string holes 11 (through holes) for strings 40 to be
passed through are formed in the frame 10 so as to penetrate from
an inner peripheral face 10a to an outer peripheral face 10b of the
frame 10. The string holes 11 are provided with an interval between
the holes in a peripheral direction of the frame 10 so as to be
arranged around substantially the entire periphery of the frame 10.
A net-shaped hitting face is formed inside the frame 10. The
hitting face is strung with plural strands of "lateral strings 41"
which are sites where the strings 40 are strung along the lateral
direction with an interval between the strings along the
longitudinal direction, and with plural "longitudinal strings 42"
which are sites where the strings 40 are strung along the
longitudinal direction with an interval between the strings in the
lateral direction.
[0042] As illustrated in FIG. 1B, a groove 12 is provided on the
outer peripheral face 10b of the frame 10 at a central portion in
the thickness direction. Openings of the string holes 11 are
provided within the groove 12, with the strings 40 being folded
back on themselves via the groove 12.
[0043] A grommet 50 is usually attached to the outer periphery of
the frame 10 (at the outside of the outer peripheral face 10b).
Such a grommet 50 includes tube-shaped (hollow circular column
shaped) string protection members 50b (see FIG. 2C), and a
belt-shaped base portion 50a (see FIG. 1) to couple the plural
string protection members 50b together. The strings 40 are strung
on the frame 10 in a state in which the grommet 50 has been
attached to the frame 10.
===Frame of Comparative Example 1===
[0044] FIG. 2A is a perspective view of a frame 10' of Comparative
Example 1 (of a leading end portion thereof). FIG. 2B and FIG. 2C
are cross-sections respectively taken at the position aa and the
position bb of FIG. 2A, so as to section the frame 10' of
Comparative Example 1 along thickness and penetration directions.
FIG. 2D is a view of an inner peripheral face 10a' of the frame 10'
of Comparative Example 1, as viewed along the penetration
direction. There is a direction normal to the outer peripheral face
10b at each of the positions where the string holes 11 are provided
on the outer peripheral face 10b of the frame 10. In the present
embodiment the string holes 11 each penetrate along the respective
normal direction (a radial direction of the hitting face), and
these normal directions are referred to as "penetration
directions". In order to avoid complicating the drawings, the
strings 40 etc. are omitted from some of the drawings, and hatching
that should be appended to cross-sections therein is sometimes
omitted.
[0045] As illustrated in FIG. 1, the strings 40 in the racket 1 are
strung in a state in which the grommet 50 has been attached to the
frame 10. Note that the grommet 50 is attached to the frame 10 by
fitting the base portion 50a into the groove 12 of the frame 10
while passing the string protection members 50b through the string
holes 11 from the outer peripheral face 10b side of the frame 10
(see FIG. 2C, FIG. 2D). The strings 40 are accordingly passed
through the string holes 11 by being passed through the base
portion 50a of the grommet and through the respective holes of the
string protection members 50b.
[0046] FIG. 3A and FIG. 3B are diagrams to explain an airflow
passing over an inner peripheral face 10a' of the frame 10' of
Comparative Example 1. FIG. 3A is a diagram illustrating the
vicinity of a string protection member 50b, as viewed along the
penetration direction. FIG. 3B is a diagram illustrating the
vicinity of the string protection member 50b, as viewed along a
peripheral direction of the frame 10'. As illustrated in FIG. 2A
and FIG. 2C, in the frame 10' of Comparative Example 1, a leading
end portion of each of the string protection members 50b projects
out from the inner peripheral face 10a' of the frame 10'.
Accordingly, as illustrated in FIG. 3A and FIG. 3B, during a swing
of the racket the airflow (indicated by the dotted arrows) passing
over the inner peripheral face 10a' of the frame 10' flows around
the string protection members 50b, namely flows around circular
columns.
[0047] Generally, when a circular column is placed in a flow, the
flow separates from the surface of the circular column so as to
generate a necklace-shaped vortex at the upstream side of the side
faces of the circular column (of the string protection member 50b),
as illustrated in FIG. 3A. Vortexes are also generated at the
downstream side of the circular column (of the string protection
member 50b), as illustrated in FIG. 3B. This is known to result in
a loss of pressure occurring at the downstream side of the circular
column, and in an increase in drag.
[0048] The air resistance acting on the racket accordingly
increases in cases such as the frame 10' of Comparative Example 1
in which the tube-shaped string protection members 50b project from
the inner peripheral face 10a' of the frame 10', and the airflow
flows around the string protection members 50b during a swing.
===Frame of Comparative Example 2===
[0049] FIG. 4A and FIG. 4B are perspective views of a frame 10'' of
Comparative Example 2 (of a leading end portion thereof). FIG. 4C
is a cross-section at position aa in FIG. 4A, illustrating the
frame 10'' sectioned along the thickness and penetration
directions. FIG. 5A is a cross-section at position bb in FIG. 4A,
illustrating the frame 10'' sectioned along the thickness and
penetration directions. FIG. 5B is a diagram illustrating an inner
peripheral face 10a'' of the frame 10'', as viewed along the
penetration direction. FIG. 5C is a cross-section at position aa in
FIG. 5B, illustrating the frame 10'' sectioned along a peripheral
direction and penetration direction of the frame 10''. FIG. 6A and
FIG. 6B are diagrams to explain an airflow passing over the inner
peripheral face 10a'' of the frame 10''.
[0050] Pairs of projections 70 are provided on the inner peripheral
face 10a'' of the frame 10'' of Comparative Example 2 at positions
in the peripheral direction of the frame 10'' overlapping the
positions where the string holes 11 are provided (for example, at
the position bb in FIG. 4A). The pairs of projections 70 are
arranged in a row along the thickness direction, as illustrated in
FIG. 5B.
[0051] The shape of the projections 70 as viewed along the
penetration direction (FIG. 5B) is a shape resembling an ellipse
cut in half along the minor axis direction, with the shape arranged
such that the major axes of the ellipse is along the thickness
direction. Namely, the shape of the projections 70 as viewed along
the penetration direction is a shape of an ellipse in which the
major axis direction is arranged so as run along the direction of
airflow during a swing. This thereby results in an aerodynamic
shape at both side faces of each of the projections 70 in the
peripheral direction.
[0052] More specifically, when each projection 70 is viewed along
the penetration direction (FIG. 5B), a peripheral direction width
W2 at a position on the central side of the projection 70 in the
thickness direction is wider than a peripheral direction width W1
at a position on the end side thereof in the thickness direction
(W1<W2). Moreover, the peripheral direction width of each of the
projections 70 widens gradually on progression in the thickness
direction from the thickness direction end side toward the
thickness direction central side, as viewed along the penetration
direction (FIG. 5B). A center of the corresponding string hole 11
in the peripheral direction is aligned with a center of each of the
respective projections 70 in the peripheral direction. The maximum
width of each projection 70 in the peripheral direction is the
diameter of the string hole 11 or greater. The string protection
members 50b accordingly do not project further out in the
peripheral direction than the projections 70.
[0053] Accordingly, as illustrated in FIG. 6A, an airflow passing
over the inner peripheral face 10a'' of the frame 10'' along the
thickness direction during a swing does not separate from the
projections 70 at the two peripheral direction side faces thereof,
and instead flows along the two side faces of the projections 70.
This enables the generation of vortexes to be suppressed. Namely,
disturbance to the airflow flowing along portions at the two sides
of the strings 40 and the string protection members 50b can be
suppressed, enabling the air resistance acting on the frame 10''
during a swing to be reduced. Moreover, providing the projections
70 causes airflow to flow along the two side faces of the
projections 70, and enables a reduction to be achieved in the
airflow that hits the strings 40 and the string protection members
50b. This may also be said to enable a reduction to be achieved in
the air resistance acting on the frame 10''. The swing speed can be
improved as a result, enabling ball speed to be increased and ball
spin amount to be increased.
[0054] As described above, FIG. 5A is a cross-section of the frame
10'' at the position bb in FIG. 4A, namely a cross-section of the
frame 10'' at a central portion of one of the projections 70 in the
peripheral direction. The peripheral direction central portion of
an upper face of the projection 70 (a face on the inner peripheral
face side in the penetration direction) has, in particular, an
aerodynamic shape such as that illustrated in FIG. 5A.
[0055] More specifically, as illustrated in FIG. 5A, a reference
position p0 is the position in the penetration direction of a
string hole 11 (opening) provided in the outer peripheral face
10b'' of the frame 10'', namely the position in the penetration
direction of a bottom portion of the groove 12. A height h2 in the
penetration direction from the reference position p0 to a
projection 70 at the thickness direction central side position of
the projection 70 at a site on the frame 10'' where the projection
70 is provided (in particular, where the peripheral direction
central portion of the projection 70 is provided) is greater than a
height h1 in the penetration direction from the reference position
p0 to the projection 70 at a position of the thickness direction
end side of the projection 70 (h1<h2). Further stated, at the
site on the frame 10'' where the projection 70 is provided (in
particular, where the central portion of the projection 70 in the
peripheral direction is provided) the height in the penetration
direction from the reference position to the projection 70
gradually increases on progression from the thickness direction end
side toward the thickness direction central side of the projection
70.
[0056] Accordingly, as illustrated in FIG. 6B, the airflow passing
along the thickness direction over the inner peripheral face 10a''
of the frame 10'' during a swing flows along the upper faces of the
projections 70 without separating from the upper faces of the
projections 70, enabling vortexes to be suppressed from being
generated. Namely, disturbance of the airflow flowing over upper
portions of the string protection members 50b can be suppressed,
enabling a reduction to be achieved in the air resistance acting on
the frame 10'' during a swing. Swing speed can accordingly be
improved as a result.
[0057] Note that, as illustrated in FIG. 5C, the projections 70
gradually increase in height in the penetration direction on
progression from the outer sides toward the central side in the
peripheral direction.
[0058] As illustrated in FIG. 4A and FIG. 4B, the projections 70
are provided at positions that overlap with the string holes 11 in
the peripheral direction of the frame 10''. Accordingly, as
described above, airflow flowing along the two side faces of the
projections 70 (FIG. 6A) enables a reduction to be achieved in the
airflow that hits the strings 40 and the string protection members
50b, enabling the air resistance to be reduced. The projections 70
are, however, not provided at positions that do not overlap with
the string holes 11 (for example, at the position aa in FIG. 4A).
However, this may be reported as not being an issue since airflow
is not disturbed thereat by the strings 40 and the string
protection members 50b.
[0059] Namely, providing the projections 70 only at positions in
the peripheral direction of the frame 10'' that overlap with the
string holes 11 reduces the number of the projections 70 while also
reducing the air resistance acting on the frame 10'' during a
swing. This enables, for example, easy manufacture of the frame
10''. However, the placement of the projections 70 is not limited
to the placement described above, and projections 70 may also be
provided at positions not overlapping with the string holes 11 in
the peripheral direction.
[0060] As illustrated in FIG. 5B, the projections 70 are provided
on the inner peripheral face 10a'' of the frame 10'' at both
thickness direction sides of the thickness direction center of the
inner peripheral face 10a''. Namely, the projections 70 are
provided in pairs. This thereby enables air resistance to be
reduced and swing speed to be improved whichever of the thickness
direction hitting faces of the racket 1 is facing in a ball-hitting
direction when swinging.
[0061] In particular, each of the pairs of projections 70 is
configured so as to have a symmetrical shape with respect to the
thickness direction center of the inner peripheral face 10a'' of
the frame 10'', enabling the same performance to be achieved on
both the front and back of the racket 1 in the thickness direction.
This thereby enables the racket 1 to be used without paying
attention to which is the front and which is the back of the racket
1.
[0062] A swing is a motion in a circular arc, and so the leading
end portion of the frame 10'' in the longitudinal direction has a
faster speed and is subject to greater air resistance when swinging
than a rear end portion of the frame 10'' in the longitudinal
direction. The air resistance acting on the leading end portion of
the frame 10'' accordingly has a large effect on swing speed. As
illustrated in FIG. 4A and FIG. 4B, providing the plural
projections 70 at only the leading end portion of the frame 10''
enables the air resistance acting on the leading end portion of the
frame 10'' to be reduced, so as to efficiently improve the swing
speed. The placement of the projections 70, however, is not limited
to the placement described above, and the projections 70 may also
be provided to portions of the frame 10'' other than at the leading
end portion.
[0063] From out of the string holes 11 provided in the leading end
portion of the frame 10'', the angles formed between the
penetration direction and the longitudinal direction are smaller
for the string holes 11 disposed at a lateral direction central
side. Even in the case where the string protection members 50b do
not project from the string holes 11, the strings 40 disposed
thereat are accordingly not prone to contacting the frame 10'' (the
edges of the string holes 11), and the strings 40 and the frame
10'' are not prone to damage. The angles formed between the
penetration direction and the longitudinal direction or the lateral
direction, however, are larger at the string holes 11 at the
outside in the lateral direction. The strings 40 thereat
accordingly bend at the inner peripheral face 10a'' of the frame
10''. The strings 40 thereat would accordingly make direct contact
with the frame 10'', causing damage to the strings 40 and the frame
10'', in the case where the string protection members 50b do not
project from the string holes 11.
[0064] Thus, as illustrated in FIG. 4B, the string protection
members 50b project further than the projections 70 at the string
holes 11 disposed at the lateral direction outsides, and the string
protection member 50b do not project further than the projections
70 at the string holes 11 disposed at the lateral direction central
side. Namely, the length of projection of the string protection
members 50b from the projections 70, for each of the string
protection members 50b that have passed through the string holes 11
overlapping in the peripheral direction with each of the
projections 70, is shorter for the projections 70 disposed more
towards the lateral direction central side from out of the
projections 70 provided at the leading end portion of the frame
10'', than for the projections 70 disposed at the lateral direction
outsides thereof. Note that as illustrated in FIG. 5A, at the
positions of the string holes 11 disposed at the lateral direction
central side, the upper faces of the projections 70 (the thickness
direction central portions of the upper faces thereof) are in the
same plane as upper faces of the corresponding string protection
members 50b.
[0065] Adopting such a configuration means that airflow does not
hit string protection members 50b projecting from the projections
70 at the string holes 11 disposed at the lateral direction central
side, enabling disturbance to the airflow to be suppressed. This
thereby enables a further reduction to be achieved in the air
resistance acting at a lateral direction central portion of the
leading end portion of the frame 10'', enabling swing speed to be
improved. At the string holes 11 on the lateral direction outsides,
however, damage to the strings 40 and the frame 10'' can be
prevented due to the string protection members 50b projecting from
the projections 70.
[0066] In this example, the projections 70 are divided at the
thickness direction center of the frame 10'', and there are no
projections 70 present at the thickness direction central portion.
There is no limitation thereto, however. For example, the
projections 70 provided at the two thickness direction sides of the
thickness direction center of the frame 10 may be integrated
together.
[0067] The projections 70 are provided in the above manner to the
inner peripheral face 10a'' in the frame 10'' of the Comparative
Example 2. This enables air resistance during a swing to be reduced
more than in the frame 10' of Comparative Example 1. However, in
the present embodiment, a further reduction in air resistance is
achieved by focusing on the flow of air at the outer periphery of
the frame during serves, strokes and the like.
Present Embodiment
[0068] FIG. 7A is a front view of a racket 1 of the present
embodiment, and FIG. 7B is a side view of the racket 1 of the
present embodiment. A grommet 50 and strings 40 are omitted from
illustration in FIG. 7A. In FIG. 7A and FIG. 7B, the positions of
the string holes 11 as counted from a top position (leading end) of
the frame 10 are given in parenthesis. For example, the string hole
11 (7) is at a location where the seventh string hole 11 is
provided, as counted from the top position of the frame.
<Frame>
[0069] In the present embodiment, projections 70 are provided on
the inner peripheral face 10a of the frame 10 of the racket 1,
similarly to in Comparative Example 2. In the present embodiment,
however, projections (projections 80) are provided to the frame 10
not only on the inner peripheral face 10a thereof, but also to the
outer peripheral face 10b thereof.
[0070] The projections 80 are provided on the outer peripheral face
10b at the leading end half of the frame 10 in a predetermined
range including a position where there is maximum curvature in the
peripheral direction. Specifically, when the inside (hitting face)
of the frame 10 is viewed as a clock face with the top position
(leading end) of the frame 10 at 12 o'clock, the projections 80 are
provided in a range from 1 o'clock to 2 o'clock, and in a range
from 10 o'clock to 11 o'clock. More specifically, the projections
80 are provided so as to correspond to the respective string holes
11 in the ranges from the string holes 11 (7) to the string holes
11 (13) on the left and right. The projections 70 on the inner
peripheral face 10a side are moreover provided so as to correspond
to the string holes 11 in ranges from the top position to the
string holes 11 (10) (the ranges where the string protection
members 50b of the grommet 50 are formed). Note that although in
the present embodiment the projections 80 are provided so as to
correspond to the string holes 11, there is no limitation thereto.
The projections 80 may also be provided so as not to correspond to
the string holes 11, as long as at least one of the projections 80
is formed in the range described above. Because providing the
projections 80 enables the rigidity of the frame 10 to be raised,
however, providing the projections 80 so as to correspond to the
string holes 11 as in the present embodiment enables the load on
the frame 10 to be reduced when the strings 40 are strung (see FIG.
1).
[0071] As illustrated in FIG. 7B, the projections 80 are provided
on both thickness direction sides of the thickness direction center
of the outer peripheral face 10b. Namely, the projections 80 are
provided on the outer peripheral face 10b of the frame 10 so as to
configure pairs on either side of the respective string holes 11.
This thereby enables air resistance to be reduced, thus enabling
the swing speed to be improved whichever of the thickness direction
hitting faces of the racket 1 is facing in the ball-hitting
direction when swinging.
[0072] In particular, each of the pairs of projections 80 is
configured so as to have a symmetrical shape with respect to the
thickness direction center of the outer peripheral face 10b of the
frame 10, enabling the same performance to be achieved on both the
front and back of the racket 1 in the thickness direction. This
thereby enables the racket 1 to be used without paying attention to
which is the front and which is the back of the racket 1.
[0073] The shapes of the projections 80 are substantially the same
as the shapes of the projections 70. Namely, both side faces of
each projection 80 in the peripheral direction have an aerodynamic
shape, with the peripheral direction width of each projection 80
gradually widening on progression in the thickness direction from
the end side toward the central side, as viewed along the
penetration direction. Namely, the width thereof in the peripheral
direction is wider at central side position in the thickness
direction than at an end side position in the thickness direction.
This thereby enables a reduction to be achieved in air resistance
irrespective of the angle of the racket 1 during a swing.
[0074] Note that in the present embodiment, a mold (not illustrated
in the drawings) for the racket 1 is formed with a pattern
corresponding to the projections 80, and the projections 80 are
formed on the frame 10 by molding the racket 1 using the mold. The
thickness of the frame 10 in the penetration direction (wall
thickness) is thus substantially the same at sites where the
projections 80 are formed and at sites where the projections 80 are
not formed. Namely, the wall thickness of the frame 10 is uniform
regardless of location. Providing the projections 80 in this manner
rather than attaching a separate member makes manufacturing easy,
and moreover enables the rigidity of the frame 10 to be raised
while suppressing an increase in weight.
<Projections at the Outer Peripheral side>
[0075] Providing projections induces flow in a boundary layer to
transition from a laminar flow to a turbulent flow, and suppresses
separation thereof. This enables the air resistance to be reduced
due to preventing the generation of large vortexes (a tripping wire
effect). The height of the projections 80 is accordingly preferably
set at the same height as the boundary layer in a laminar flow
boundary layer, or slightly lower than the height of the boundary
layer.
[0076] Generally, when a viscous substance is flowing at some
speed, a distinction may be made between a portion thereof where
the viscosity may be ignored (a portion distanced from the racket 1
in this case), and a portion thereof that is affected by viscosity
(a portion near to the racket 1 in this case). The boundary layer
is the portion affected by viscosity. Taking .delta. to indicate
the height of the boundary layer, then .delta. is expressed by the
following Equation 1 for a boundary layer in laminar flow.
.delta.=5.0.times.(kinematic viscosity.times.distance from object
edge/speed).sup.1/2 Equation 1
Namely, the height of the boundary layer is dependent on the square
root of the distance from the object edge.
Wherein
[0077] kinematic viscosity of air: 15 mm.sup.2/s at 20.degree.
C.
[0078] swing velocity: 30 m/s (108 km/h)
[0079] distance from object edge (20 mm)
[0080] Substituting the above values in Equation 1 gives
.delta.=0.5 mm. The swing velocity is the swing speed for an
upper-intermediate level tennis player. The distance from the
object edge is the frame thickness (length from the thickness
direction edge) for face-on direction, and is the distance between
projections for peripheral length direction. Accordingly, setting
the height of the projections 80 to 0.5 mm or lower enables the
flow in the boundary layer to be made turbulent and airflow
separation to be suppressed, as described later (see FIG. 12B),
thereby enabling air resistance to be reduced. Conversely, were the
height of the projections 80 to be set greater than the boundary
layer .delta. then this might disturb flow including flow outside
the boundary layer, resulting in the formation of large vortexes
and an increase in resistance.
[0081] Note that this calculation for a professional level swing
speed of 40 m/s yields .delta.=0.43 mm. Based on these
calculations, the height of the projections 80 is set in the
present embodiment to approximately 0.4 mm, this being slightly
below .delta..
[0082] An appropriate projection width is from approximately 3 mm
to approximately 8 mm, and is preferably 5 mm, in order to achieve
a combination of a degree of smoothness that avoids the projections
breaking and damage to other objects as a result of contact with
the racket, while also maintaining the effectiveness of the
projections to disturb flow so as to induce a tripping wire
effect.
<Grommet>
[0083] FIG. 8 is a diagram to explain a grommet 50 of the present
embodiment. FIG. 9 is a diagram including a cross-section taken
along A-A and a cross-section taken along B-B in FIG. 8. The
circled numbers in FIG. 8 correspond to the numbers of the string
holes 11 in the frame 10 (the numbers in parenthesis in FIG. 7)
when the grommet 50 has been placed on the frame 10. Namely, the
length direction (i.e. peripheral direction) center of the grommet
50 is disposed at the top position (the leading end) of the frame
10, and the two string protection members 50b closest to the center
are inserted through the string holes 11 (1).
[0084] FIG. 10A is a front view of a leading end portion of the
racket 1 of the present embodiment in a state in which the grommet
50 has been attached to the frame 10 of the racket 1. FIG. 10B is a
perspective view thereof. FIG. 11A and FIG. 11B are cross-sections
taken along A-A in FIG. 10A. FIG. 11A is a cross-section
illustrating the frame 10 alone, and FIG. 11B is a cross-section
illustrating a state in which the grommet 50 has been attached to
the frame 10.
[0085] As described above, the grommet 50 includes the tube-shaped
(hollow circular column shaped) string protection members 50b and
the belt-shaped base portion 50a to couple the plural string
protection members 50b together. The base portion 50a of the
grommet 50 in the present embodiment also includes protrusions 51
and recesses 52.
[0086] The recesses 52 are provided on a back face (inside face) of
the base portion 50a, this being a face on the side opposing the
frame 10. The recesses 52 are provided with indented shapes so as
to fit together with the projections 80 on the outer periphery of
the frame 10 when the grommet 50 has been attached to the frame
10.
[0087] The protrusions 51 are provided on a front face (outside
face) of the base portion 50a, this being a face on the opposite
side to the back face. The positions where the protrusions 51 are
formed correspond to the positions where the recesses 52 (and the
projections 80 of the frame 10) are formed. The shape of the
protrusions 51 is a similar shape to that of the projections 80 of
the frame 10 (i.e. an aerodynamic shape). This enables a reduction
in air resistance similar to that of the frame 10 alone to be
achieved when the grommet 50 has been attached to the frame 10.
[0088] Note that the recesses 52 and the protrusions 51 of the
grommet 50 in the present embodiment are provided at positions
corresponding to the respective string holes 11 of the frame 10
from the string holes 11 (7) to the string holes 11 (11), as
illustrated in FIG. 8. Thus when the grommet 50 has been attached
to the frame 10, the projections 80 at positions up to the string
holes 11 (11) of the frame 10 are covered by the grommet 50.
Conversely, the projections 80 at positions of the string holes 11
(12) and the string holes 11 (13) are not covered by the grommet
50, and are therefore exposed (see FIG. 10A, FIG. 10B).
[0089] In this manner, there are in the present embodiment both
sites where the projections 80 of the frame 10 are exposed and
sites where the projections 80 are not exposed when the grommet 50
has been attached to the frame 10. There is no limitation thereto,
however, and, for example, a configuration may be adopted in which
none of the projections 80 of the frame 10 are exposed (i.e. all
are covered by the grommet 50). In cases in which the projections
80 are not exposed, a configuration may be adopted in which the
projections 80 are not provided to the frame 10 and there are only
protrusions 51 provided to the grommet 50. For example, the grommet
50 of the present embodiment may be attached to the racket 1 of
Comparative Example 1 or Comparative Example 2. In such cases there
may be recesses 52 present at positions corresponding to the
protrusions 51, or the recesses 52 may be omitted. Providing the
recesses 52 enables a reduction in weight to be achieved.
<Flow of Air at Frame Outside>
[0090] FIG. 12A is a concept diagram illustrating a flow of air at
the outside of the frame 10' of Comparative Example 1. FIG. 12B is
a concept diagram illustrating a flow of air at the outside of the
frame 10 of the present embodiment (with the grommet 50 attached
thereto).
[0091] As illustrated in FIG. 12A, in Comparative Example 1, after
the airflow has passed sites with a large curvature, the airflow is
then liable to separate at the rear half. This results in large
vortexes and increases resistance. Similar also applies to
Comparative Example 2 (the frame 10'').
[0092] As illustrated in FIG. 12B, due to providing the projections
80 (and the protrusions 51 of the grommet 50) in the present
embodiment in the leading half that includes sites with a large
curvature, the projections create turbulent flow within the
boundary layer, thereby making the airflow less prone to separating
and resulting in smaller vortexes on the downstream side. This
thereby enables a reduction in air resistance to be achieved at the
frame outer peripheral side in comparison to Comparative Example 1
and Comparative Example 2.
[0093] Note that the positions where the projections 80 are
provided may be sites with a large curvature, as described above
(in other words, sites where the radius of curvature is small).
This is because the pressure change at sites with a small curvature
is gentler and the airflow is not liable to separate when in the
wind, and there is accordingly a low necessity to provide
projections 80 thereat. Conversely, the pressure change is sudden
at sites with a large curvature and the airflow is more liable to
separate when in the wind, leading to large vortexes such as those
illustrated in FIG. 12A being generated and an increase in
resistance. Thus providing the projections 80 in a range including
the sites with a large curvature as in the present embodiment
enables separation of airflow to be suppressed, and also enables
vortexes to be made smaller, enabling a reduction in air resistance
to be achieved.
===Racket Evaluation Test===
[0094] FIG. 13A and FIG. 13B are explanatory diagrams illustrating
a test method to evaluate air resistance acting on the racket 1.
FIG. 13B is a diagram of the racket 1 as viewed from above (from
the leading end side thereof). FIG. 14A and FIG. 14B are diagrams
showing the results of evaluation tests.
[0095] As illustrated in FIG. 13A, equipment for the evaluation
test includes a wind tunnel 90, a support stand 91 to support the
racket 1 subject to evaluation, and a load cell 92. The wind tunnel
90 blows wind against the entire region of the frame 10 of the
racket 1. The support stand 91 supports the frame 10 of the racket
1 such that the racket 1 is capable of pivoting in response to the
wind from the wind tunnel 90. The load cell 92 is attached to the
upstream side of the racket 1 in the wind direction at a site on
the shaft 20 further to the grip 30 side than the pivot point of
the racket 1. The load cell 92 measures the force attempting to
move the site on the shaft 20 further to the grip 30 side than the
pivot point in the opposite direction to the wind direction when
the racket 1 is pivoted by the wind from the wind tunnel 90.
Namely, the load cell 92 measures the reaction force of the racket
1 to the wind from the wind tunnel 90. The value measured by the
load cell 92 is taken as the air resistance acting on the racket
1.
[0096] Air resistance was measured using the evaluation test
equipment described above for the racket of Comparative Example 2
and for the racket of the present embodiment. Note that position P1
in FIG. 13B, which is one in which a blower outlet of the wind
tunnel 90 and the hitting face of the frame are parallel to each
other, is taken as a reference position. An angle .theta. formed
between the hitting face of the frame 10, 10'' at varying tilts and
the hitting face of the frame at the reference position is referred
to as the angle of attack. For example, the angle of attack .theta.
is 30.degree. when the frame 10, 10'' is at position P2 in FIG.
13B, the angle of attack .theta. is 60.degree. when the frame 10,
10'' is at position P3, and the angle of attack .theta. is
90.degree. when the frame 10, 10'' is at position P4. The air
resistance was measured while setting the speed of wind from the
wind tunnel 90 and varying the angle of attack of the frame 10,
10'' in a range of from 0.degree. to 90.degree..
[0097] First, the air resistance was measured while varying the
wind speed in a fixed state of a 15.degree. angle of attack .theta.
between the direction of the wind and the hitting face of the
frame. The results are illustrated in FIG. 14A. The horizontal axis
in FIG. 14A indicates wind speed (m/s), and the vertical axis
indicates air resistance (N).
[0098] As the figures illustrate, the rate of increase in
resistance declines in the present embodiment from around the 20
m/s wind speed mark, and in comparison to Comparative Example 2, a
reduction in air resistance of approximately 20% can be achieved at
wind speeds of 23 m/s or greater (corresponding to the swing speed
of an ordinary lower-intermediate level player).
[0099] Moreover, as illustrated in FIG. 13B, the air resistance was
also measured while varying the tilt (the angle of attack .theta.)
of the frame with respect to the direction of the wind. These
results are illustrated in FIG. 14B, and are shown as proportional
reductions in air resistance of the present embodiment in
comparison to the air resistance of Comparative Example 2 for each
of the angles of attack .theta..
[0100] As the figures illustrate, the air resistance was lower
overall than Comparative Example 2 in each case, and in particular
the results obtained indicate there to be a high air resistance
reduction effect for angles of attack .theta. of from 15.degree. to
30.degree..
Other Embodiments
[0101] The above embodiment modes are to facilitate understanding
of this invention, and are not for limiting this invention in any
way. It is needless to say that this invention can be changed or
modified without deviating from the scope, and this invention
includes its equivalents.
[0102] For example, although in the embodiment described above a
tennis racket is given as an example, there is no limitation
thereto. For example, the invention may also be applied to a squash
racket, a badminton racket, or the like. Moreover, although in the
embodiment described above a racket having strings strung in a
frame is given as an example, there is no limitation thereto, and a
racket not strung with strings may be employed.
[0103] Although in the embodiment described above the projections
(the projections 70 and the projections 80) are respectively
provided at the inner peripheral side and the outer peripheral side
of the frame 10, a configuration lacking the projections 70 at the
inner peripheral side may be adopted. For example, the projections
80 may be provided to the outer peripheral face 10b' of the frame
10' of Comparative Example 1. Such cases would also enable a
reduction in air resistance to be achieved, and higher rigidity to
be achieved, in comparison to the frame 10' of Comparative Example
1.
REFERENCE SIGNS LIST
[0104] 1: racket; 10: frame; 10a: inner peripheral face; 10b: outer
peripheral face; 11: string hole; 12: groove; 20: shaft; 30: grip;
40: string; 41: lateral string; 42: longitudinal string; 50:
grommet; 50a: base portion; 50b: string protection member; 51:
protrusion; 52: recess; 70: projection (inner peripheral side); 80:
projection (outer peripheral side); 90: wind tunnel; 91: support
stand; 92: load cell.
* * * * *